1. Field of the Invention
This invention relates to a light beam scanning system which causes a light beam to scan a recording medium in order to record image information on the recording medium or to read the image information recorded on the recording medium.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, u-rays, .alpha.-rays, .beta.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
Recently, the stimulable phosphor sheet has been put into use in various fields in addition to the medical field such as the radiation image recording. For example, this applicant has proposed a method of recording and reproducing electron microscopic images by the use of the stimulable phosphor sheet. See U.S. Pat. No. 4,651,220 and U.S. patent application No. 251,982 now U.S. Pat. No. 4,889,990, and the like. The method basically comprises the steps of exposing a stimulable phosphor sheet to cathode rays which have passed through a specimen under vacuum in order to store a cathode ray image on the stimulable phosphor sheet, scanning the stimulable phosphor sheet with stimulating rays thereby causing the stimulable phosphor sheet to emit light in proportion to the amount of energy stored during exposure to the cathode rays, photoelectrically detecting the light thereby obtaining an electric image signal, and reproducing the electron image of the specimen as a visible image on the basis of the image signal.
In the read-out apparatus which detects the light emitted from the stimulable phosphor sheet upon stimulation thereof in order to reproduce the electron microscope image, it is preferred that the stimulating rays scan the stimulable phosphor sheet at a higher density and the image is detected with higher accuracy than in that of the aforesaid radiation image recording and reproducing system. However, the conventional read-out apparatus which has been used in the radiation image recording and reproducing system cannot satisfactorily meet these requirements. That is, in the typical radiation image recording and reproducing system, a light beam deflected in a main scanning direction by a light deflector is caused to enter a main scanning lens system, which may comprise a f.theta. lens, and is focussed on a stimulable phosphor sheet which is conveyed in a sub-scanning direction at a constant speed. In order to limit the size of the scanning lens system, the f.theta. lens must have a relatively long focal length and must be at a distance from the stimulable phosphor sheet. Due to the relatively long focal length of the f.theta. lens, the diameter of the stimulating light beam of the stimulable phosphor sheet cannot be sufficiently small.
This applicant has proposed in U.S. patent application No. 225,443 (now U.S. Pat. No. 4,886,968), a light beam scanning system which is particularly preferable as a read-out apparatus for the system for carrying out the aforesaid method of recording and reproducing electron microscopic images. The light beam scanning system comprises a sheet holding means having a cylindrical sheet support surface on which the stimulable phosphor sheet is held, and a spinner having a light deflector mirror which reflects a stimulating light beam on the central axis of the cylindrical sheet support surface toward the stimulable phosphor sheet on the sheet support surface and a condenser lens system which converges the reflected stimulating light beam on the stimulable phosphor sheet. The stimulating light beam is caused to scan the stimulable phosphor sheet in the main direction by rotation of the spinner, and the sub-scanning is effected by relative movement of the sheet holding means to the spinner.
In this light beam scanning system, the condenser lens system need not be enlarged in size even if it is disposed close to the stimulable phosphor sheet since the main scanning of the stimulating light beam is effected by the rotation of the spinner. Accordingly, the condenser lens system may have a short focal length which converges the stimulating light beam into a small spot on the stimulable phosphor sheet, which permits a high-density read-out of the image information.
However, in the light beam scanning system, there is a problem in that the stimulable phosphor sheet is apt to lift away from the cylindrical sheet support surface of the sheet holding means since the stimulable phosphor sheet which is flat is conveyed onto the cylindrical surface, held thereon and conveyed out therefrom. More particularly, if the stimulable phosphor sheet is rigid due to the large thickness thereof or low environmental temperature, the stimulable phosphor sheet can lift away from the support surface when the stimulable phosphor sheet is conveyed onto the support surface. Though this may be avoided by a pair of guide members which are respectively provided on opposite ends of the support surface and press the stimulable phosphor sheet against the support surface, this approach is disadvantageous in that the end portions of the stimulable phosphor sheet is covered with the guide members and the image information stored on the whole surface of the stimulable phosphor sheet cannot be read out. If the stimulable phosphor sheet is held against the support surface by suction means instead of the guide members, the image information stored on the whole surface of the stimulable phosphor sheet can be read out. However in this case, if the stimulable phosphor sheet lifts significantly away from the stimulable phosphor sheet, the suction means cannot attract the stimulable phosphor sheet. Further, if the suction force provided to the suction means is cut off due to power failure or the like during the read-out operation, the stimulable phosphor sheet will abruptly lift away from the support surface and will interfere with the spinner which is rotating at high speed, which can result in damage to the spinner and/or the stimulable phosphor sheet.
The problems described above are encountered not only in the light beam scanning system in which the stimulable phosphor sheet is employed as the recording medium but also in other light beam scanning systems in which other recording medium such as a photosensitive recording sheet is employed.